Originally Posted by
treadigraph
There are several turbine aircraft that can often be seen and heard orbiting areas of London at around 9000' and which don't appear on the radar apps. Most people with an ear for aviation can likely pick em out but I imagine the population in general would be hard pressed to notice them over the general hubbub of the metropolis. I can tell you that they are jolly hard to spot if the air below them is less than gin clear! Easier on days when there is some form of altostratus...
Sounds like Big Brother is watching you!!.. ('corse that would never happen over here: our guvmint is too busy wasting money on train tunnels no-one needs to fund an operation like that)